1. Field of the Invention
The present invention relates to a wire electrical discharge machine provided with a rotating shaft and capable of stably machining a rotating workpiece.
2. Description of the Related Art
Japanese Utility Model Application Laid-Open No. 2-97524 and Japanese Patent Application Laid-Open No. 7-136853 disclose wire electrical discharge machines designed so that a workpiece is rotated as a wire electrode is moved for desired machining, such as machining of a spiral groove in a high-hardness collet-chuck assembly that is used to fix a tool, e.g., a drill. A collet-chuck is a holding device that holds a material by its entire cylindrical inner surface so that the material cannot be easily damaged.
FIG. 19 is a diagram illustrating how a workpiece 1 is rotated as a wire electrode 2 is moved for desired machining by a wire electrical discharge machine. As shown in FIG. 19, desired machining is performed in such a manner that the workpiece 1 is rotated in a workpiece rotation direction 5 about a rotation center axis 6, and at the same time, the wire electrode 2 is moved relative to the workpiece 1 in a wire-electrode movement direction 4. A numerical controller for controlling the wire electrical discharge machine simultaneously issues move commands to a rotation axis about which the workpiece 1 is rotated and a linear axis along which the wire electrode 2 is relatively moved.
While the numerical controller thus simultaneously issues the move commands to the rotation axis for the rotation of the workpiece 1 and the linear axis for the relative movement of the wire electrode 2, control for the speed of movement is conventionally performed in the following manner.
(a) The linear axis (wire electrode) moves at a command speed, and the rotation axis (for workpiece rotation) follows the movement of the wire electrode.
(b) Control is performed such that a synthetic speed of the linear and rotation axes agrees with the command speed, regarding a movement command unit (deg) of the rotation axis as equivalent to that (mm) of the linear axis.
In either of the cases (a) and (b), the speeds of movement of the linear and rotation axes are constant. However, the distance (hereinafter referred to as “rotation radius”) between the center of rotation and a position (workpiece machining point) where the workpiece 1 is machined varies with the progress of machining of the workpiece. In some cases, the rotation radius at the workpiece machining point may change for each block of a machining program or gradually vary from the start point of one block toward the end point.
FIG. 20 is a diagram illustrating how the distance from the center of rotation of the workpiece 1 corresponding to the workpiece machining point to the machining point varies as machining of the workpiece progresses. With the progress of the workpiece machining, the distance (rotation radius) from the center of rotation of the workpiece to the workpiece machining point changes from r1 to r2, as shown in FIG. 20. There is a problem that the rotation radius varies between workpiece machining points 7a and 7b and workpiece machining points 8a and 8b for machining by the wire electrode 2, so that the speed of relative movement of the wire electrode 2 and the workpiece 1 changes, thus resulting in a change in the electrical discharge state and unstable machining. As a countermeasure against this, there is a conventional method in which each machining program is divided into very small movement blocks and a command speed is calculated and commanded such that the speed of relative movement of the wire electrode 2 and the workpiece 1 is constant for the blocks. There is a problem, however, that the calculation of the command speed and the creation of machining programs are time-consuming and the machining programs increase.
As shown in FIG. 21, moreover, there may be a problem that the thickness of a workpiece that is hollow inside and varies in inside diameter and wall thickness also changes during machining, thus resulting in a change in the electrical discharge state and unstable machining.